Inspection Technique for CUI Sumitomo Chemical …...SUMITOMO KAGAKU 2010-I 3 Inspection Technique for CUI (Corrosion under Insulation) by Using Fiber Optical AE Sensor acteristics

1SUMITOMO KAGAKU 2010-I

Introduction

1. Handling of Corrosion Under Insulation in

Japan and Overseas

Corrosion under insulation (CUI in the following)

started getting close attention in chemical plants, petro-

leum refineries, power generating facilities and other

plants in the developed nations in Europe and the

Americas as well as Japan in the 1980s. This was due to

the fact that insulation that had previously been used in

locations at 149°C or higher started being used in tem-

perature ranges of 100°C or lower after the first oil cri-

sis in 1973. In Japan, the High Pressure Gas Safety

Institute of Japan issued a report on external corrosion

in 1988, and the Engineering Advancement Association

of Japan (ENAA in the following) carried out an investi-

gation into the application of nondestructive testing

techniques and monitoring techniques from 2007

through 2011. According to a 2007 ENAA report,

guided wave ultrasound, real time radiography and

pulsed eddy current nondestructive inspection tech-

niques have been developed and applied at working

sites, but they are not yet complete. Therefore, estab-

lishing priorities and visual inspection where the insula-

tion is peeled off of the entire length of all surfaces are

recommended.1)

2. Handling of Inspections at Sumitomo Chemical

The following shows how inspections have been han-

dled by the Sumitomo Chemical Process & Production

Technology Center since 2007. In 2007, we fabricated a

mock-up pipe (pipe simulation) as shown in Fig. 1 and

conducted an investigation into applying detection of

corrosion swelling using a fiber optic acoustic emission

(AE in the following) sensor. Starting in 2008, we began

working on the development of environmental noise

separation techniques that included on-site applica-

tions. In 2008, we also investigated insulation structures

for corrosion resistance (exterior plate structures), and

we have carried out investigations into the selection of

optimal undercoating materials for a temperature range

of 60°C to 100°C. We continued these investigations in

2009, but we will discuss the status of development of

inspection methods using fiber optic AE sensors here.

Inspection Technique for CUI(Corrosion under Insulation) byUsing Fiber Optical AE Sensor

Sumitomo Chemical Co., Ltd.

Process & Production Technology Center

Toyokazu TADA

Hidehiko SUETSUGU

Hisakazu MORI

Corrosion under insulation (CUI) is one of the degradation phenomena that have become a serious problem inrecent years especially in chemical plants that have been operating for a long time. Development of a CUI inspectiontechnique which doesn’t require the removal of insulation and which is applicable to explosion-proof petrochemicalplants is strongly needed. So we focused attention on optical fiber Doppler sensors which already have the explo-sion-proof characteristics, and we tried to develop a new CUI inspection technique using them. The developmentof this new inspection technique is explained.

Fig. 1 Photo of the mock-up pipe

This paper is translated from R&D Report, “SUMITOMO KAGAKU”, vol. 2010-I.


Inspection Technique for CUI (Corrosion under Insulation) by Using Fiber Optical AE Sensor

Development of CUI Inspection Techniques

Using Fiber Optic AE2)

CUI in carbon steel equipment and pipes has become

a deterioration phenomenon that has intensified in re-

cent years particularly in chemical plants that have

been operating for many years. In particular, exterior vi-

sual inspection of pipes installed outside in pipe racks

that are in high places is difficult, and also, since the

total distance is large, no CUI inspection methods have

been established that are more effective than visual in-

spection with the insulation removed. In addition, the

fact that 70 to 80% of the costs for these inspections is

scaffolding and tearing down the insulation is one major

problem. Therefore, there was a strong requirement for

development of CUI inspection techniques for pipes

that did not require the work of removing the insulation

and worked for plant facilities requiring protection from

explosions. We focused on fiber optical Doppler (FOD

in the figures) sensors, which have explosion preven-

tion properties from the beginning and attempted to de-

velop new CUI inspection techniques.

1. Comparison and problems in CUI inspection

methods for conventional pipes

Table 1 gives the characteristics of various inspection

methods that have been applied to pipes up to now.

Methods that have good inspection accuracy can only

carry out inspections at short distances, and methods ca-

pable of long-distance inspections (approximately 5m)

have poor inspection accuracy. Therefore, most opera-

tions carry out inspections where the insulation is re-

moved as described previously, but even if complete tear-

down inspections are carried out at great expense, corro-

sion is only found in 2 and 3 systems out of every 1000

systems. The problem is that the efficiency is very poor.

2. Principles of AE monitoring using fiber optical

Doppler3)

We focused on the relationship between corrosion

and AE to establish an inspection method that was ef-

fective for pipe CUI. Fig. 2 is a conceptual diagram of

the AE generating mechanism. First of all, peeling and

cracking of local corrosion products (corrosion

swelling) occur because of the progress in active corro-

sion. At this time, the strain energy that has accumu-

lated inside is released as minute elastic waves. Since

these elastic waves are comparatively low frequency

elastic waves from the audible to 500kHz, it is known

for propagating in a wide range. Therefore, the pres-

ence of corrosion can be sensed by detecting the AE

caused by the peeling and cracking of the corrosion

using an AE sensor to detect the elastic waves as AE. In

other words, we can consider improving the CUI in-

spection efficiency by tearing down the insulation and

carrying out visual inspections only on pipes where AE

has been observed.

The AE method is superior for monitoring corrosion,

and it is already being applied in some places such as

evaluation of corrosion damage on the bottoms of

tanks, but there are actually various of problems. Up to

now the AE method has used piezoelectric sensors, but

there are problems such as: (1) they are easily affected

by wind, vibration, noise, abrasion noise from the fluid

inside, etc. (2) If attempts are made at increasing the

sensitivity of the sensor, the range of measurement be-

comes a narrow band, so separation is impossible when

it overlaps with a noise frequency band. (3) Since the

resistance to electromagnetic noise is poor in cables,

use for long distances is impossible. (4) They do not

Fig. 2 Image of AE technique

Crack…

Corrosion Tuberclesof Iron Rust

AE (audible sound~about 500kHz)

AE sensor

Table 1 Comparison of the CUI inspection technique for pipes

Influence of internal fluidInspection of long distance

Removal of insulations

Corrosion can be detectedInspection accuracy

YES

Not Applicable

No need

localized corrosion(φ2mm through

wall hole)

Good

Inspection method

Real-Time RT

Radio graphic Testing (RT)

NO

Not Applicable

No need(Less than 80mm

in thickness)General

corrosion like concave

Not good

Pulsed ECT

Eddy Current Testing (ECT)

YES

Applicable

Need

localized corrosion(10% depth level of

sectional area)

Not good

Ultrasonic guided waves

Ultrasound Testing (UT)



acteristics of being able to obtain an output proportional

to the rate of expansion and contraction of the fiber,

having a broad receiving band of 1 Hz to 1 MHz and

having a broad temperature range of –200°C to 250°C.

In addition, advantages of fiber optical Doppler sen-

sors include (1) having high insulation properties, (2)

having high resistance to electromagnetic noise, (3)

having explosion prevention properties and not generat-

ing electrical sparks, (4) being capable of long distance

measurements and (5) having a broad range of applica-

ble environments. Therefore, disadvantages (2)

through (5) for the conventional piezoelectric AE sen-

sors described previously can in principle be resolved

by using fiber optical Doppler sensors.

Fig. 5 is a conceptual diagram of a fiber optic AE

monitoring system for pipe CUI. First, frequency ƒ0

lightwaves from a laser Doppler vibrometer light

source in a measurement circuit are introduced into the

to the optical fiber. The frequency of the lightwaves that

are incident to the fiber optical Doppler sensor receive

the AE arising because of the corrosion flaking and

cracking and are modulated to ƒ0 through ƒd. On the

other hand, the amount of frequency modulation is de-

tected using the heterodyne interferometric method.

Specifically, a reference light with frequency ƒM

(80MHz) is added using a frequency modulator

(AOM), creating ƒ0 + ƒM modulation. Furthermore, the

difference in frequency ƒM + ƒd of the laser light from

the sensor and the laser light from the measurement

circuit is introduced and ƒd is detected. It is converted

to voltage V by a frequency-voltage converter and out-

put. A frequency analysis (FFT) is used on the original

waveform data obtained at this time, and it is converted

into extracted data such that the horizontal axis is fre-

quency and the vertical axis is spectral power. This

waveform analysis is important as a technique for dis-

criminating noise and the AE caused by corrosion.

have explosion prevention properties. (5) The range of

temperature applications is limited. To solve these

problems, attention has been given to the fiber optic AE

technology that has been developed in recent years.

The image of telecommunications systems is strong

for fiber optics, but optical fibers can be used as sensors

by using their Doppler effect. Now, when light waves

from light source with acoustic velocity C and fre-

quency ƒ0 are incident to an optical fiber, we let the opti-

cal fiber be extended length L at velocity v (see Fig. 3).

Letting frequency ƒ0 be changed to ƒ1 because of the

Doppler effect at this time, frequency ƒ1 following the

change can be expressed as in Eq. 1 by the formula for

the Doppler effect.

Therefore, if we consider modulation of frequency ƒ1

following the change from frequency ƒ0 prior to inci-

dence by ƒd, we get Eq. 2.

ƒd can be expressed as in Eq. 4 using the formula for

the wave given in Eq. 3.

Eq. 4 shows that the rate of expansion and contrac-

tion of the optical fiber can be detected as and fre-

quency modulation of the lightwave.

In other words, we can detect the strain (elastic

waves, changes in stress, etc.) applied to the optical

fiber by reading the frequency modulation ƒd of the opti-

cal fiber. Fiber optical Doppler sensors have been de-

veloped for sensors that make use of the Doppler effect

when these optical fibers expand and contract (see Fig.

4). To increase the sensitivity of these sensors and

make reception from all directions possible, the optical

fiber is wound into a coil; therefore, the it has the char-

(Eq. 1)f1 = f0 = f0 – · f0CC – v

Cv

(Eq. 2)fd = f0 · Cv

(Eq. 4)fd = f0 · · · v =λ

=C

1dtdLv

Cf0

(Eq. 3)C = f0 · λ

Fig. 3 Model of Doppler effect of optical fiber

Optical fiber

f0 v f1 = f0 – fd

Light source

Fig. 4 Principle of FOD sensor

Sensor(fixing)

N: number of turns

f 0 – fd

f0

out

in

N



3. Mock-up piping for investigating CUI inspec-

tions

Up to now there have been actual records of fiber

optic AE evaluations of areas with loosening base rock

that accompanies underground storage tank excava-

tion, but there are no records of use in CUI evaluations.

Therefore, we fabricated mock-up piping like that

shown in Fig. 6 to investigate the possibilities of devel-

oping CUI inspection technology using fiber optic AE.

Insulation was applied to a 5m carbon steel pipe, and

heated silicon oil was circulated inside the pipe by a

heating device. In addition, corrosion was accelerated

artificially by pure water and salt where the amount

dripped was finely adjusted to an extent giving rise to

wetting and drying to efficiently generate CUI and fur-

ther by heating to 60 to 70°C using silicon oil. A fiber

optical Doppler sensor is positioned at any location on

this mock-up piping and data collected regarding AE

from this corrosion site.

In this investigation, we used a commercial layered

fiber optical Doppler sensor that was a 65m optical fiber

wound into a layered coil shape (see Fig. 7). This sen-

sor is characterized by having an increased sensitivity

because the optical fiber is installed in a layered form.

Even though it has a broad band, it has the same or

greater receiving sensitivity than a narrow band con-

ventional piezoelectric AE sensor. When the layered

fiber optical Doppler sensor is installed on the piping, it

Fig. 5 Image of optical fiber of CUI

–6000

–4000

–2000

2000

0

4000

6000

0 500 1000 1500 20000 50000 100000 150000 2000000.00E+00

5.00E+07

1.00E+08

1.50E+08

2.00E+08

2.50E+08

3.00E+08

Fast Fouriertransform (FFT)

Time (µsec)Frequency (Hz)

Pow

er s

pect

ral (

a.u.

)

Vol

tage

(m

V)

Original data Sampled data

The noise and AE are sorted.

Light source

Optical fiber

FOD Sensor

HM

Acousto-OpticModulator (AOM)

HM

Measurement circuit (Laser Doppler Vibrometer)

AE caused by flaking of rust

Output Voltage

(80MHz)

Corrosion

Detector

f0

f0 – fd

f0 f0

f0

f0 – fdf0 + fM

fM + fd

fd

Fig. 6 Mock-up piping for CUI inspection investigation

70°C

Silicon oil heating device in piping

pure water + salt + 70°C heating

Accerelate the corrosionintentionally

Insulation Carbon steel pipingFOD sensor



was an abrupt increase in AE after the start of the pure

water and salt dripping and after the increase in temper-

ature. In addition, the AE converges after a certain

amount of time, and the AE increases when the oil tem-

peratures lowered. The increase in the number of AE

hits when the wetting and drying and changes in tem-

perature are applied in this manner is a major character-

istic of AE.

In addition, the AE waveforms recorded at this time

are grouped into three patterns, ones that have frequen-

cies exceeding 100kHz, ones at 50 to 100kHz and ones

at 10 to 50kHz as shown in Fig. 10. These results can be

said to be due to the fiber optic AE being able to receive

a broad band. In measurements on actual equipment, it

is important to selectively receive the AE in a frequency

band from these three patterns of frequencies that does

not overlap measurement noise. In addition, the lower

the frequency for the AE, the further the propagation is,

but since it is easy for management noise to arise on the

low frequency side, it is necessary to measure the AE

is secured using a U-bolt, and when it is installed on a

flange, it is secured with a clamp. Either is extremely

easy to install and remove, and like piezoelectric AE

measurements, sending and receiving is possible

through a commercial contact medium.

4. Results of measurements with mock-up piping4)

(1) Results of AE detection investigations at the initial

stages of corrosion

We carried out tests investigating AE detection in the

initial stages of corrosion in November 2007. The state

of corrosion on the piping at this time is shown in Fig.

8. Corrosion swelling has yet to arise, and the parts that

look white are salt that has recrystallized. The fiber op-

tical Doppler sensor was installed at a position 300mm

from the site of the corrosion. The AE detection results

are shown in Fig. 9. The bar graph shows the number

of AE hits per hour, and the plotted line shows the cu-

mulative AE hits.

This proves that AE measurements are possible at

initial stages of corrosion like this. In addition, there

Fig. 8 Corrosion of piping (1st monitor: 11/2007)

Fig. 9 AE hits by initial corrosion

0

10

20

30

40

50

60

70

80

1129

1111

2912

1129

1311

2914

1129

1511

2916

1129

1711

2918

1129

1911

2920

1129

2111

2922

1129

2311

300

1130

111

302

1130

311

304

1130

511

306

1130

711

308

1130

911

310

1131

111

312

1131

311

314

0

50

100

150

200

250

300

350

400AE hits/1 hour Total AE hits

AE

hits

/1 h

our (

hits

)

Tota

l AE

hits

Measurement date

PureWaterSalt

decrease in temperature

increase in temperature

Fig. 7 FOD sensor installation

Piping

Fixed by U-shaped bolt

Fixed by screw clamp

Flange FOD sensor

FOD sensor

Install and remove easily



3,900mm from the site of the corrosion. Fig. 12 shows

the AE detection results at the 3,900mm position. In

this manner we confirmed that it was possible to detect

AE at a sufficient sensitivity even at maximum distance

of 3,900mm. In addition, we found that the 50k to

100kHz frequencies were detected the most out of the

three AE patterns. If no noise is present in this range of

frequencies in measurements on actual equipment,

which are a problem we will take up later, we can effi-

ciently detect AE caused by corrosion.

pattern at as low a frequency as possible that does not

overlap with the noise.

(2) Results of investigations into distances where AE is

detectable

In January 2008 we conducted verifications of the dis-

tances at which AE could be detected. The state of cor-

rosion on the piping at this time is shown in Fig. 11.

Corrosion swelling has arisen, and in the progress of

the corrosion can be seen. We installed the fiber optical

Doppler sensor at positions 2,000mm, 3,000mm and

Fig. 10 Data of original AE and Frequency spectrum pattern

Peak above 100kHz

Peak between 50k-100kHz

Peak between 10k-50kHz

–6000

–4000

–2000

2000

4000

6000

0.00E+00

5.00E+07

1.00E+08

1.50E+08

2.00E+08

2.50E+08

3.00E+08

–800–600–400–200

200400600800

1000000

2000000

3000000

4000000

5000000

6000000

7000000

–6000

–4000

–2000

2000

4000

6000

0.00E+00

5.00E+07

1.00E+08

1.50E+08

2.00E+08

2.50E+08

3.00E+08

00 500 1000 1500 2000 50000 100000 150000 200000

0

0

0

0

Time (µsec) Frequency (Hz)

0 50000 100000 150000 200000Frequency (Hz)

0 50000 100000 150000 200000Frequency (Hz)

Pow

er s

pect

ral (

a.u.

)Po

wer

spe

ctra

l (a.

u.)

Pow

er s

pect

ral (

a.u.

)

Vol

tage

(m

V)

0 500 1000 1500 2000Time (µsec)

Vol

tage

(m

V)

0 500 1000 1500 2000Time (µsec)

Vol

tage

(m

V)

Fig. 11 Corrosion of piping (2nd monitor: 01/2008) Fig. 12 AE hits by corrosion progress at 3,900mm

Distance 3,900mm

100k-150kHz 50k-100kHz 10k-50kHz

Elapsed time (minute)

AE

hits

/30

min

utes

(hits

)

0

10

20

30

40

50

60

0-30 30-60 60-90 90-120 120-150 150-180 180-210 210-240



(3) Comparison of AE detection results on the piping

and flange parts

In March 2008 we compared AE detection on the pip-

ing and flange parts. The state of corrosion on the pip-

ing at this time is shown in Fig. 13. The corrosion

swelling has grown further, and cracking heads ap-

peared in part of the corrosion swelling. Fiber optical

Doppler sensors were installed on the pipe at 3,900mm

and on a flange part at 3,950mm from the site of the cor-

rosion is shown in Fig. 14.

Fig. 15 shows a comparison of the results for AE de-

tection on the piping and flange parts. While these re-

sults show poorer sensitivity than on the pipe itself, we

confirmed that there was excellent AE detection on the

flange part. If it is possible to detect AE on the flanges,

we can expect to only remove the insulation from the

flange parts to make AE measurements.

(4) Investigation of corrosion progress and number of

AE hits

To make a comparison with the number of AE hits

from the corrosion site in January 2008 and the number

of AE hits from the corrosion site in March 2008 after

the corrosion had progressed further, we installed a

fiber optical Doppler sensor at the same 3,900mm posi-

tion and compared the results of measurements. The

results of that comparison are shown in Fig. 16. How-

ever, the AE measurements in January 2008 were only

made up to 240 minutes. As is clear from this figure, we

can see that the number of AE hits in March has clearly

increased with the progress in the corrosion compared

with those in January (an approximately tenfold AE hit

number at 240 minutes). The AE method cannot quan-

tify the area of the corrosion or depth of material reduc-

tion, but if the volume of the corrosion swelling

increases, the probability of AE hits increases. There-

fore, we can consider the possibility of evaluations

using the degree of correlation with the progress of the

corrosion by counting the number of AE hits.

Fiber Optic AE Measurements in Actual Equip-

ment5)

1. Selection of stationary equipment and environ-

mental noise separation

As was discussed earlier, it is possible to inspect

Fig. 15 Comparison of the situation of AE hits piping and flange

0

50

100

150

200

250

300

350

400

450

500

0

500

1000

1500

2000

2500

3000

30-6060-90

90-120120-150

150-180180-210

210-240240-270

270-300300-330

330-3600-30

AE

hits

/30

min

utes

(hits

)

Tot

al A

E h

its


AE hits/30 minutes (on piping) AE hits/30 minutes (on flange) Total AE hits (on piping) Total AE hits (on flange)

Fig. 13 Corrosion of piping (3rd monitor: 03/2008)

Fig. 14 The position of FOD sensor installation

FOD sensor

3,900mm

3,950mm

Fig. 16 Comparison of the number of AE hits 01/2008 and 03/2008

0

50

100

150

200

250

300

350

400

450

500

0

500

1000

1500

2000

2500

3000

30-6060-90

90-120120-150

150-180180-210

210-240240-270

270-300300-330

330-3600-30

AE

hits

/30

min

utes

(hits

)

Tot

al A

E h

its


AE hits/30 minutes (03/2008) AE hits/30 minutes (01/2008) Total AE hits (03/2008) Total AE hits (01/2008)



As is shown in Fig. 19, the fiber optical Doppler sen-

sors were protected by a waterproof case. After the sur-

face coating on the outside of the reactor was removed

with sandpaper, they were glued using a heat resistant

epoxy resin adhesive and secured from the top using

aluminum tape. The signal cables extending from the

fiber optical Doppler sensors were each collected in a

jack box as ch1, ch2, ch3 and ch4 (see Fig. 20), and a

cable was extended from there to the ground. When AE

measurements are made, a terminal box installed on

the aboveground part and a fiber optical Doppler inter-

ferometer installed in a vehicle are connected by a cable

(see Fig. 21). One of the major merits of this method is

that, once the fiber optical Doppler sensors are con-

nected to the actual equipment, inspections can easily

pipes without tearing down insulation even if it cannot

be called complete, but there is no method for equip-

ment other than tearing down insulation and visual in-

spection. Therefore, we selected an operating vertical

reactor (3.8m inside diameter, column length approxi-

mately 28m) where CUI might be revealed as the equip-

ment to be targeted for measurements. There was a

plan for the insulation to be removed from the entire

surface of the column in this equipment and to perform

the work of eliminating the corrosion that had arisen

due to CUI by surface preparation. Therefore, fiber opti-

cal Doppler sensors were installed along with this work,

and after the corrosion had been removed, AE measure-

ments were made. We investigated the separation of

the AE from corrosion under conditions that included

the environmental noise due to the fluid inside. More-

over, if a CUI inspection method that is more efficient

than visual inspection is established for stationary

equipment like vertical reactors, installation of scaffold-

ing and tearing down the insulation will become unnec-

essary, which has great merit in terms of costs.

2. Attachment of fiber optical Doppler sensors to

vertical reactor

The positions for attaching the fiber optical Doppler

sensors to the vertical reactor are shown in Fig. 17 and

Fig. 18. Four (four channels) fiber optical Doppler sen-

sors were attached at intervals of approximately

3,000mm with a pitch of 90° around the circumference

positioned vertically at 9,500mm based on the girth line

for the lower mirror and body of the main body of the

reactor.

Fig. 18 The position of FOD sensor installation and wiring for signal cable

FOD sensor ch4

FOD sensorch1

FOD sensorch2

FOD sensorch3

N

Junction box

Ladder

φ4,000mm

AE monitorsystem

Fig. 17 Structure of the vertical reactor

A–A'

A A'

Heat exchanger

Pass partition

5500

9500

13500

1440

17500

Heat exchanger

Heatexchanger

Air nozzle

Fig. 19 The condition of FOD sensor installation on the vertical reactor



be carried out from the aboveground part which is at a

distance from the sensors.

3. AE measurement results before and after repair

of corroded parts

The results of AE measurements before and after the

removal of the corrosion due to CUI that had arisen on

the equipment by surface preparation are shown in Fig.

22 and Fig. 23, respectively. As a result, a large amount

of AE was detected in the state where the corrosion due

to CUI was present, and after the corrosion had been re-

moved, the number of AE hits was reduced to approxi-

mately 1/10. Therefore, we assumed that we were also

able to detect AE that can be thought of as being caused

by corrosion on actual equipment. In addition, there

was also a large difference in the number of AE detec-

tions by each of the sensors before removal of the cor-

rosion. In visual inspections after the insulation was

removed, multiple corrosion sites were found to be

present in the vicinity of the sensors where there were

many AE detections. In this manner, we were able to

confirm that there was a good correlation between the

state of corrosion and the state of AE detection in tests

carried out using actual equipment.

However, the AE shown in Fig. 23, which was de-

tected after the removal of the corrosion, is not caused

by corrosion. It can be assumed to be environmental

noise that could not be eliminated by filtering using cur-

rent waveform processing techniques. Being able to

separate out this environmental noise as precisely as

possible is extremely important for using this technol-

ogy at actual sites.

4. Investigation of environmental noise separation

The typical AE waveforms detected before and after

the removal of the corrosion and the AE waveforms

from corrosion that were obtained in the investigations

with the CUI mock-up piping are shown in Fig. 24

through Fig. 26, respectively. Since the rise of the

waveforms, the duration, etc. have a very similar shape,

it can be assumed that separating the environmental

noise out using the shape of the waveforms will be diffi-

cult.

Next, the frequency distributions for the peak fre-

quency (frequency with the largest amplitude in the fre-

quency spectrum) and the barycentric frequency

(frequency that is the center of gravity for the area of

the frequency spectrum) for a measurement time of

three hours of AE detected before and after the removal

Fig. 21 AE measurement condition in which FOD interferometer and collection analysis device are loaded into car

FOD interferometer

AE recorder

Fig. 23 AE hits/30 minutes after removing rust

0

50

100

150

200

250

ch1 ch2 ch3 ch4

11:00–11:30 12:00–12:3011:30–12:00 12:30–13:00 13:00–13:30 13:30–14:000–30 60–9030–60 90–120 120–150 150–180

AE

hits

/30

min

utes

(hits

)

Fig. 22 AE hits/30 minutes before removing rust

0

50

100

150

200

250 ch1 ch2 ch3 ch4

10:05–10:35 10:35–11:05 11:05–11:35 11:35–12:05 12:35–13:050–30 30–60 60–90 90–120 150–180

AE

hits

/30

min

utes

(hits

)

Fig. 20 Connection of 4ch FOD sensor to junk BOX



of the corrosion are shown in Fig. 27 and Fig. 28. Both

of the frequency bands are substantially the same as the

corrosion AE frequency band (10kHz to 150kHz) ob-

tained in the investigations with the CUI mock-up pip-

ing. In addition, since the frequency distributions for

the AE peak frequency and the barycentric frequency

before and after the removal of the corrosion have very

similar distribution shapes, it can be assumed that envi-

ronmental noise separation by frequency analysis will

be difficult.

Next, the frequency distributions for the maximum

amplitude and number of AE hits for a measurement

time of three hours of AE detected before and after

the removal of the corrosion are shown in Fig. 29 and

Fig. 30, respectively. There are many small maximum

amplitudes for AE before the removal of the corrosion,

and as the maximum amplitudes increase, the number

of AE hits decreases. On the other hand, while the

shape of the distribution for AE after removal of the cor-

rosion is roughly similar to the AE before removal, a

clearly significant difference can be seen in this fre-

quency distribution instead of the small maximum am-

plitudes being the most common. Therefore, we

attempted a comparison using the m value (shape pa-

rameter for the distribution function) to make a more

easily understood numerical conversion for these re-

sults. The m value is the slope of the distribution graph

line obtained when two axes in the graphs for the fre-

quency distributions of the maximum amplitude and

hits for AE shown in Fig. 29 and Fig. 30 are shown log-

arithmically. The corrosion AE m values obtained for

the AE detected before and after the removal of the cor-

rosion and in the investigations with the CUI mock-up

piping are shown in Fig. 31.

Fig. 25 Representative Waveform of AE obtained after removing rust

–1000

–800

–600

–400

–200

0

200

400

600

800

1000

Time (µsec)

Vol

tage

(m

V)

0 500 1000 1500 2000

Fig. 26 Representative Waveform of AE obtained from mock-up piping

–6000

–4000

–2000

0

2000

4000

6000

Time (µsec)

Vol

tage

(m

V)

0 500 1000 1500 2000

Fig. 24 Representative Waveform of AE obtained before removing rust

–500

–400

–300

–200

–100

0

100

200

300

400

500

Time (µsec)

Vol

tage

(m

V)

0 500 1000 1500 2000

Fig. 28 Frequency distribution of AE obtained after removing rust

0

20

40

60

80

100

120Peak FrequencyBarycentric frequency

~50 50–100 100–150 150–200 200–250 250–300 kHz

AE

hits

/3 h

ours

(hi

ts)

Frequency distribution

Fig. 27 Frequency distribution of AE obtained before removing rust

0

500

1000

1500

2000

2500Peak FrequencyBarycentric frequency

Frequency distribution

~50 50–100 100–150 150–200 200–250 250–300 kHz

AE

hits

/3 h

ours

(hi

ts)



In these results, the m values for AE before and after

the removal of the corrosion were –2.23 and –1.71, re-

spectively, and the m value for the AE from the corro-

sion in the CUI mock-up piping was –2.67. It was found

that the AE frequency distribution before the removal

of the corrosion was very similar to the AE frequency

distribution from the corrosion on the CUI mock-up pip-

ing and that the frequency distribution of the AE after

the removal of the corrosion differed. These results

suggested that it would be possible to separate AE due

to corrosion and the pseudo-AE due to the environmen-

tal noise by finding the m value.

However, to use this separation technique at actual

sites, we will require the accumulation of more funda-

mental data based on laboratory results and the accu-

mulation of separation data for AE due to corrosion and

environmental noise through the accumulation of tests

with actual equipment on site.

Summary

The AE arising from corrosion swelling was captured

roughly in the frequency range of 10kHz to 150kHz. In

addition, it was shown that AE could be captured even

at a maximum length of 4,000mm. This shows that it is

possible to inspect a range of 8,000mm left him right

with a single sensor, and since there was plenty of mar-

gin in the sensitivity, we can assume that even longer

distances can be inspected. In addition, even if the sen-

sor was installed on a flange part, it was possible to cap-

ture the AE from the pipe part in the same manner.

Furthermore, AE that was thought to be caused by cor-

rosion could also be detected in actual equipment, and

it was suggested that separation of corrosion AE from

environmental noise would be possible by carrying out

evaluations of the m values (shape parameter). Since

fiber optic AE sensors have explosion prevention from

the beginning, normal installation of the sensor parts

may also be performed in plants that have where explo-

sions must be prevented such as petrochemical plants.

We hope to accumulate more data in the future and

move toward making this practical rapidly.

References

1) S. Hara, K.Yamamoto, Bosei kanri, 53 (3), 106

(2009).

2) T. Tada, H. Mori, H. Cho, Y. Machijima, JSNDI fall

conference 2008, 243 (2008).

3) K. Kageyama, H. Murayama, K. Uzawa, I. Ohsawa,

M. Kanai, Y. Akematsu, K. Nagata and T. Ogawa,

Journal of Lightwave Technology, 24, 1768 (2006).

4) Sumitomo Chemical Co., Ltd, Patent applying.

5) Sumitomo Chemical Co., Ltd, Patent applying.

Fig. 30 Distribution of AE hits and maximum amplitude obtained after removing rust

100

400

700

1000

1300

1600

1900

2200

2500

2800

3100

3400

3700

4000

4300

4600

4900

5200

5500

5800

0

5

10

15

20

25

30

35

40

AE

hits

/3 h

ours

(hi

ts)

Maximum amplitude (mV)

Fig. 31 Comparison by m value (Shape parameter)

Mock-up pipingBefore remove rust After remove rust

log

AE

hits

Log Maximum amplitude (mV)

“m” means the slope of approximate straight-line

y = –2.6719x + 10.183

y = –2.2302x + 8.5901

y = –1.7142x + 6.0529

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Fig. 29 Distribution of AE hits and maximum amplitude obtained before removing rust

0

100

200

300

400

500

600

100

400

700

1000

1300

1600

1900

2200

2500

2800

3100

3400

3700

4000

4300

4600

4900

5200

5500

5800

AE

hits

/3 h

ours

(hi

ts)

Maximum amplitude (mV)



P R O F I L E

Toyokazu TADA

Sumitomo Chemical Co., Ltd.Process & Production Technology CenterResearcher

Hisakazu MORI

Sumitomo Chemical Co., Ltd.Process & Production Technology CenterResearcher

Hidehiko SUETSUGU

Sumitomo Chemical Co., Ltd.Process & Production Technology CenterSenior Research Associate

Documents

Inspection Technique for CUI Sumitomo Chemical …...SUMITOMO KAGAKU 2010-I 3 Inspection Technique for CUI (Corrosion under Insulation) by Using Fiber Optical AE Sensor acteristics